1. Field of the Invention
The present invention relates to a conductive paste and a glass circuit structure comprising a conductive film formed on a glass substrate by using the conductive paste. Particularly, the present invention relates to a conductive paste and a glass circuit structure suitably used for a defogging glass for an automobile window.
2. Description of the Related Art
In an automobile rear window glass, for example, a heating conductor is disposed at the glass, for defogging or defrosting (simply referred to as “fogging prevention” or “defogging” hereinafter) so that the heating conductor is heated by passing a current through a conductor circuit including the conductor to keep the glass surface temperature at the dew point or higher.
In many cases, this heating conductor comprises a plurality of strips and bus bars connected to both ends of the strips formed on a glass surface. The heating conductor having such a structure comprises a conductor film formed by printing a conductive paste in a pattern on the glass surface and then baking the paste. Also, a lead-out metal terminal is connected to each of the bus bars by soldering or the like to form the conductor circuit in which the heating conductor is heated by applying a voltage between the metal terminals.
Since a constant-voltage power supply is used for applying the voltage applied the metal terminals, the quantity of heat generated from the heating conductor depends upon the shape, including the thickness, and the resistivity (volume resistivity) of the heating conductor. Therefore, it is desirable to realize conductors having various resistivity values from a material aspect in order to control the quantity of heat generated.
For example, a resistance adjuster is mixed with a conductive paste containing a silver powder, a low-melting-point glass frit and an organic vehicle, for adjusting the resistance of the paste to a desired value by controlling sintering, thereby adjusting the resistivity of a conductor (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2000-48642 and 9-92028).
As the resistance adjuster, for example, a metal having high resistivity, such as Ni, Al, Sn, Pb, Pt, Pd or the like, or an oxide thereof, is used.
However, when the resistance adjuster is used, the metal terminals cannot be connected with sufficiently high strength in some cases. A possible reason for this is that when the above-described metal or oxide is added as the resistance adjuster to the conductive paste, the adjuster is solid-dissolved in glass frit in the burning process to inhibit flow of glass, thereby decreasing the bonding force between the heating conductor and a glass substrate. Another possible reason is that the amounts of oxides, which are not involved in soldering, in the sintered conductor film are increased to segregate the oxides near the surface of the conductor film, and the segregated oxides inhibit solderability to decrease the strength of bonding between the metal terminals and the heating conductor.
It is thought that in order to decrease the amount of the metal or oxide added, which is likely to cause the above problem, the properties such as the shape, particle diameter and crystallinity of the silver powder contained as a conductive component in the conductive paste can be changed to decrease the sinterability of the sliver powder. However, this countermeasure causes roughening of the silver sintered texture to easily produce cohesive failure in the conductor film and decrease the strength of bonding with the glass substrate.